Recent advances in ultra-low power wireless communications and energy-harvesting technologies have made self-sustainable devices feasible. Typically, the major concern for these devices is battery life and replacement. Applying energy harvesting techniques to these devices can significantly extend battery life and sometimes even eliminate the need for a battery.
Ambient energy is available in different forms (i.e. small vibrations, light, temperature gradients, electromagnetic waves, etc). This energy that is all around us can be converted to electrical energy with current technologies like solar cells, piezoelectric devices, thermoelectric devices, or special antennas. This paper aims to discuss one of the available techniques: RF energy harvesting.
Ambient energy in the form of man-made electromagnetic radiation is abundant regardless of the system's location (i.e. indoor or outdoor system) or time of day.  In contrast, systems that rely on thermal gradients or solar power might have special location considerations.
Major communication and broadcasting systems are serviced in the frequency bands between 75 MHz to 3.0 GHz, including TV, FM Radio, mobile phone and WiFi.  The trick lies in picking the frequency band in this spectrum with the most power and designing around it. A rectenna is a rectifying antenna that can be used to convert the electromagnetic radiation to electricity (RF-DC). 
Recently, a research group at Georgia Tech created a battery-less system capable of scavenging energy around the 470-570 MHz frequency band. Their scavenging circuit managed to trickle charge a super capacitor to 1.8, 2.2 and 3.6 V, in accordance with current microcontroller power needs.  Their final goal is to use this super capacitor to power a microcontroller and a wireless transceiver, thereby creating a truly autonomous wireless sensor.
In previous work, members of the same group managed to create an RFID-enabled Temperature sensing device. This device operated in the 2.4 GHz frequency band and achieved a maximum range of 8 meters. After calibration, the best results yielded an accuracy of -0.8 to +0.6 °C.  The system under inspection in this experiment required 0.5 mW of power for transmission.
These energy harvesters are built with the idea of creating a Wireless Sensor Network of truly autonomous devices. The transmission of information between these devices is the main power consumer for the application in mind. A group at Columbia University has been working on new low power wireless communication protocols. Using pulses on the order of nano-seconds, data has been successfully transmitted and received at a mere 2.5 nJ/bit, and 43 pJ/bit, respectively at a data rate of 17 Mpulses/s.  The group is currently working on novel algorithms that allow the individual sensors in the network to adapt their communication rates to save energy depending on their stored power at a given time.
Significant amount of research is being conducted in the area of energy harvesting. There are various ways of harvesting the ambient energy, but one that has received a lot of attention lately is the RF energy harvesting technique. Under this scheme, power is harvested from electromagnetic radiation coming from major broadcasting systems like mobile phones, TV, radio, or WiFi. The main application for energy harvesting at the moment is the creation of wireless sensor networks which depend on truly autonomous devices that require no intervention after the installation stage. In order for this scheme to work, the sensors must rely on new ultra-low power communication protocols that allow them to make the best use of the small amounts of ambient energy they have at their disposal.
© Alvaro Aguilar. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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